A first fluid circuit of a cooling loop is commonly referred to as a “hot leg” because it transports the water drawn from the vessel to the steam generator, and a second fluid circuit is commonly called a “cold leg” because it returns the water cooled by the steam generator back to the vessel. The tubes of a steam generator frequently have an inverted U-shape and are commonly called “hairpins”. These tubes are immersed in the water of a closed secondary circuit which circulates in the steam generator, in order to cause this water to boil and supply steam to a turbine driving a generator. By design, steam generators typically contain several hundred hairpins, each formed by a tube about a centimeter in diameter. The tubes are generally located above the level of the vessel and constitute high points which cannot be vented. This portion of the primary circuit therefore requires a special procedure to fill it.
Each of the units of a nuclear power plant must periodically be shut down for maintenance operations and to replenish the nuclear fuel. It is necessary to prevent large air pockets from remaining trapped in the steam generators when refilling. When the reactor is restarted, although the volume of the air pockets greatly decreases due to pressurization of the primary circuit in order to maintain the water in the liquid state by preventing it from boiling, these air pockets circulate through the circuit with the water and excessively large air pockets could damage the primary pumps due to the cavitation phenomenon caused by the passage of a large volume of air through a pump. As a result, and also for chemical reasons, it is necessary to eliminate almost all the air contained in the primary circuit before the unit is restarted.
In order to fill the primary circuit with water so that the volume of trapped air remains below a threshold imposed by safety regulators according to the type of facility, there exist several methods for filling with water and emptying the air from the primary circuit of a nuclear unit after it has been shut down to replenish the fuel. A first method, commonly called “dynamic venting”, consists of simply filling the hairpin tubes with water when the pumps of the primary circuit are restarted. The air thus expelled from the hairpins accumulates at the high points of the primary circuit, such as the lid of the vessel, which are then vented. This operation is repeated multiple times, then an air assessment is conducted. This filling and venting method requires numerous pressurizations and depressurizations of the primary circuit, which can be harmful to the mechanical resistance of the vessel in the long term, and also requires starting and stopping the primary pump a significant number of times.
In order to prolong the service life of the power plants, another filling method is used in which the filling is performed after creating a vacuum in the primary circuit, with the nuclear fuel being in place within the vessel. This method has disadvantages, however. In particular, during the depressurization phase and filling phase, the vacuum must be maintained in the primary circuit. If air intake occurs or if a failure occurs in the installation which creates the vacuum, the operations must be restarted. In addition, the preparations for establishing a vacuum are relatively long, typically a hundred hours, several of these hours being on the critical path in the shutdown schedule for the nuclear unit.
The invention aims to overcome the above disadvantages, and in particular it aims to provide a method for filling with water and emptying the air from a primary circuit of a nuclear reactor in preparation for restarting it, which is less costly in terms of management time and which decreases the length of the facility shutdown while complying with nuclear safety rules.